Geochemical prospecting method



Aug. 27, 1946. H. 'r. KENNEDY GEOCHBIICAL PHOSPEGTING METHOD Fllod July 3. 1940 2 Shanta-Shut 2 Patented Aug. 27, 1946 uui rao srA'rss PATENT OFFICE Gulf Research a Development 0cm burgh. 2a., a corporation of Delaware Application July 3, 19M, Serial No. 348.804

method of prospecting for gas and oil deposits including the steps of taking samples of ground water in the earth at various spaced points at levels adjacent the water table and quantitatively determining the concentration of petroleum hydrocarbons in the samples; the method being also applicable to other deposits giving rise to water soluble vapors or gases; all as more fully hereinafter set forth and as claimed.

Oil and gas prospecting methods have been devised involving detection and quantitative measurement of traces of oil and gas hydrocarbons in the earth at various points over a region, with the immediate object of locating and deilning regions of high gas concentration and the ultimate object of locating subterranean oil-and gas reservoirs. Such geochemical prospecting methods are attractive because they are more direct than the usual geophysical prospecting methods which at best can only reveal favorable geological structures which may or may not contain oil and gas.

The earliest geochemical methods involved taking samples of gas directly from the soil near the surface or at depth. However such procedures often give very erratic results. In taking a sample of gas by suction through a pipe more or less air is drawn in from the surrounding earth. and the amount of dilution from this source depends on the permeability and other characteristics of the surrounding earth. By the same token such a gas sample is not fairly representative of a small definite zone but may include gas from other levels or leaking in through crevices from quite distant points.

Some improvement is had by taking samples of the soil or rock itself, and extracting gas therefrom for analysis. Porous soil entrains gas and also acts as an adsorbent for gas in a manner somewhat like activated charcoal. But here the concentrations formed depend on the porosity and adsorptivity of the soil, which may vary widely from place to place. and on other factors.

In all these. methods the concentrations of oil and gas hydrocarbons (mostly methane and ethane) are extremely small; of the order of a few parts per million at most and often only a few parts per billion.

The present invention is based on the discovery that by taking samples of ground waters at various points over a region to be investigated, and determining the (minute) concentrations of oil and gas hydrocarbons therein, the meas- 2 Claims. (01. 23-230) ,ured concentrations bear a far closer relation to the true distribution of the hydrocarbons in the earth, and to the oil and gas deposits from which the hydrocarbons difluse, than in any of the previous expedients. Among other things, errors due to variations in adsorptivity of soils are obviated.

Ground water is meteoric water (rainfall, melting snow, etc.) which has percolated downward into the pores and fissures oi the superficial portion of the earth. Ground water saturates the earth from an upper level, known as the water table, downward as far as free percolation is possible. It is distinguished from water or brines associated with, and of age corresponding to. the deeper sedimentary rocks. Ground waters are ordinarily fresh, that is free of dissolved salines,

while connate waters are often brines. Ground water moves in slow currents through the earth. under the influence of gravity, and makes its appearance at springs and at water wells.

It is this ground water; as distinguished from connate waters. that is sampled and analyzed for petroleum hydrccarbons according to the invention. The ground water has in general not come into contact with oil or gas pay formations; it is utilized in the present invention merely as an absorbent or reservoir for seep gases which have come from such. formations.

In carrying out the invention, test wells to the water table are dug at various points over a region under investigation, if water wells are not already available. and water samples are taken under conditions preservative of the gas content thereof. Gas concentrations are measured and plotted on a map of the region to show gas highs and "lows. It is desirable to know the direction of movement of the ground waters, as an aid to tracing structure. This can be determined by suitable methods or can be inferred from local geological data.

It is known that leakage or diffusion of the more volatile petroleum hydrocarbons through the crust of the earth takes place at all times although the amount of such transfer may be exceedingly minute. In some localities, vertical diilusion may be of suilicient magnitude so that source of the emanation, which may be a fault, a Joint. or other channel through which the emanation may reach the surface, especially by making a series of tests in the general direction of movement of the ground water. Since the complete characteristics of faults are frequently known or determinable by surface or geophysical measurements, it is often possible to determine with considerable accuracy the ultimate source of the emanation.

The water samples taken as described are freed of gas by heating and the gas is analyzed by any suitable method capable of the required high sensitivity. Suitable spectrographic and microcombustion methods are available. but at present I regard a fractional evaporation method described in detail below as the most convenient.

In the accompanying drawings, there are shown diagrammatically an illustration of field practice of the invention and a diagram of one suitable apparatus organization for analysis.

In the drawings:

Fig. 1 is a schematic view in central vertical section through the earth, illustrating the relation of ground water gas concentrations to structure in a typical case.

Fig. 2 is a diagrammatic showing of one good apparatus organization suitable for use in the invention. and

Fig. 3 is a chart showing trations are determined.

Referring to Fig. 1, a section of the earth I about four miles lon is shown schematically, with a buried fault 6 of which there is no visible indication at the surface of the earth. The water table is indicated at I. Below this surface the earth i saturated with water, flowing in the direction shown. G'as seeps up through the fault as shown by arrows. The situation diagrammed is typical of the Gulf Coast. Water samples are taken at wells 0. 8, l and II and at point l2 where the water table almost intersects the surface of the earth. Samples are generally not taken in open bodies of water, as these are subject to atmospheric actions and moreover may be contaminated with marsh gas of vegetable origin. The samples, substantially free of soil, are quantitatively analyzed for ethane or other petroleum hydrocarbon, and the concentration values (usually a few parts in ten million) are plotted to scale. The abrupt rise in ethane concentration at l3, considered in relation to the direction of ground water movement. gives an indication of the fault.

Referring to Fig. 2, illustrative of th analysis organization, 20 is a sample tube which may be filled with ground water by pumping the water through the tube, or by lowering the tube into the well, or by any other suitable means. In some cases it is desirable to employ special sample containers which may be sealed while in the well, thus preventing any loss of emanation. A convenient sample size is a liter or thereabouts. The method of examining water samples will be explained with particular reference to the determination of ethane, since this constituent seems to be the most reliable indicator for petroleum. To accomplish the analysis for ethane in ground water the sample of ground water in the container 20 is displaced by mercury up to the stopcock 2! in the neck of the still 20. The water sample in the still is heated by heater 2 and the water is boiled and recondensed by the condenser II. Gases which were dissolved in the water thus how ethane concenconcentrate in the upper vapor space of the condenser and are drawn slowly over through the water trap 20 which is maintained at about 80 C. by a bath or dry ice in a Dewar container 21 a surrounding it. 'Ihe gases are then drawn through a drier tube moisture are removed by passage over phosphoric anhydride, and thence pass through a tube II filled with caustic soda, where carbon dioxide and other acid gases are removed. A Toepler pump 30 of conventional form draws the gases through the preceding purification train and delivers them to a reservoir Ii from which they flow at a slow and uniform rate through an activated-charcoal tube 32 immersed in a dry ice bath maintained at a temperature oi about -80 C. The reservoir II is adapted to accumulate gases removed from the water in the early stages of the pumping when gas is being pumped at a rapid rate and to deliver these gases slowly and uniformly through the charcoal tube 82. Reservoir I8 is a level bottle which can be moved ver tically to receive and withdraw mercury from reservoir II when the pinch clamp is open. During the process of passage of gas through the charcoal tube 32 the hydrocarbons are adsorbed on the activated charcoal and the residual gas consisting mainly of oxygen, nitrogen. and sometimes methane. pass through the charcoal and are vented to the atmosphere by means of the two-way cock ll. It has been found that the removal of or per cent of the water in the still 28 to the trap insures the complete removal of ethane from the water sample, provided the water is boiling vigorously with ample reflux in the condenser 20. When this amount of water has been collected in the trap 28 the stopcock 35 is closed and the tubes I0, 28 and 20 are exhausted to a pressure of a fraction of a millimeter of mercury by continued action of the Toepler pump 30 transferring substantially all of the dried and purified vapors removed from the water sample to the charcoal tube 32. When all the gases have been passed through the charcoal tube, excess air in the charcoal and in the apparatus to the right of this tube are exhausted by turning the cock ll so as to lead to a mercury pump (not shown in the diagram) the cock I! being closed and the mercury cut-ofls 80 and 39 being lowered. When the pressure in the system I2, 80, I0 and 40 reaches about 0.002 mm. mercury as measured by the McLeod gauge ll about three grams of mercury are admitted into the charcoal tube 32 from the reservoir 3| by raising the reservoir 33 and heat is applied to the charcoal tube 32 to vaporize the mercury, removing adsorbed gases from the charcoal and carrying these gases into the vapor trap 40 where ethane and less volatile vapors are condensed, co the mercury in the meantime being condensed in the cut-oil 8B and residual air and other volatile gas are pumped off. When the mercury distillation is complete the adsorbed gases are entirely removed from the charcoal, this being done at a temperature of about 150 C. The mercury level in the cut-off 80 is then raised, the cock 31 opened to the mercury pump again until the pressure in the system 40, 4| and 88 is lowered to about 0.002 mm. The temperature or the vapor trap 40 is then slowly raised by allowing the bath surrounding it to warm up, the temperature being conveniently measured by a tripleiunction thermocouple attached to gpotentiometer circuit (not shown) and the pressure measured by the McLeod gauge ll. Both tem- 20 where the last traces of y 5 perature and pressure are measured at frequent intervals as shown in Figure 3.

The data obtained are plotted as shown in Fi ure 3 and the pressure rise DP between the two breaks is measured. The volume 01' ethane in milliliters at atmospheric pressure originally present in the water sample is then determined by multiplying the pressure rise in atmospheres by the volume in milliliters of the apparatus from the top of the mercury on th right side of the mercury cut-oi! I. to the top of the mercury on the left side of the cut-oi! 39, including the Mo- Leod gauge. By this method, ethane in amounts as small as 0.0001 milliliter may be detected, and larger amounts may be measured with a fair degree of accuracy. Other emanations such as methane, propane may be similarly determined, but for reasons already given. ethane is preferred as an indicator of petroleum.

What I claim is:

1. A method of prospecting a region of the earth for subterranean gas and oil deposits which comprises taking samples of ground water in the earth at a plurality of spaced points in the region, at levels adjacent the water table, said ground water samples bein free from significant quantitles of soil, confining the samples from access to air before substantial diminution of ethane concentration therein, and thereafter quantitatively determining traces of ethane dissolved in the samples as indicative of the presence oi oil or gas.

2. A method of exploring for relatively deep petroleum deposits which comprises taking a ground water sample substantially free of soil particles from each or a plurality of spaced points located near the water table in the zone of exploration. promptly confining each sample from contact with extraneous fluids and tly analyzing the samples for dissolved gaseous constituents derivable from petroleum, as indicative of the presence of relatively deep hydrocarbon deposits.

HARVEY '1'. 1

Certificate of Correction Patent No. 2,406,6 1 1.

August 27, 1946.

HARVEY T. KENNEDY It is hereby certified that error appears in the num ered patent requiring correction as follows:

rinted specification of the above olumn 4, line 58, strike out the to a low temperature such that; and that the therein that the same may Oifice.

LESLIE FRAZER,

First Am'atmzt Uommiasioner of Patents.

5 perature and pressure are measured at frequent intervals as shown in Figure 3.

The data obtained are plotted as shown in Fi ure 3 and the pressure rise DP between the two breaks is measured. The volume 01' ethane in milliliters at atmospheric pressure originally present in the water sample is then determined by multiplying the pressure rise in atmospheres by the volume in milliliters of the apparatus from the top of the mercury on th right side of the mercury cut-oi! I. to the top of the mercury on the left side of the cut-oi! 39, including the Mo- Leod gauge. By this method, ethane in amounts as small as 0.0001 milliliter may be detected, and larger amounts may be measured with a fair degree of accuracy. Other emanations such as methane, propane may be similarly determined, but for reasons already given. ethane is preferred as an indicator of petroleum.

What I claim is:

1. A method of prospecting a region of the earth for subterranean gas and oil deposits which comprises taking samples of ground water in the earth at a plurality of spaced points in the region, at levels adjacent the water table, said ground water samples bein free from significant quantitles of soil, confining the samples from access to air before substantial diminution of ethane concentration therein, and thereafter quantitatively determining traces of ethane dissolved in the samples as indicative of the presence oi oil or gas.

2. A method of exploring for relatively deep petroleum deposits which comprises taking a ground water sample substantially free of soil particles from each or a plurality of spaced points located near the water table in the zone of exploration. promptly confining each sample from contact with extraneous fluids and tly analyzing the samples for dissolved gaseous constituents derivable from petroleum, as indicative of the presence of relatively deep hydrocarbon deposits.

HARVEY '1'. 1

Certificate of Correction Patent No. 2,406,6 1 1.

August 27, 1946.

HARVEY T. KENNEDY It is hereby certified that error appears in the num ered patent requiring correction as follows:

rinted specification of the above olumn 4, line 58, strike out the to a low temperature such that; and that the therein that the same may Oifice.

LESLIE FRAZER,

First Am'atmzt Uommiasioner of Patents. 

